A note about the purples and yellows...I notice they go through social cycles, at least in my system. At times they will pick on one, not let it out of the caves, etc. I have seen one yellow that I thought would buy the farm he got so skinny, and then one day he got tired of it and started coming out.
I really don't fully understand what they are up to of course, but each fish in the group has at one time or another fallen into dis-favor. This will generally last for a few days to a couple weeks. And I know the purples have a reputation for being aggressive, but my yellows are just as bad if not worse. I do see somewhat of a correlation to water changes. In other words, if I skip a couple, they seem to get more rambunctious. Just an observation here.
Oddly enough, PBTs seem to have the same reputation, but mine is a model citizen. I surmize from my experience that it really is down to individual personality and group dynamics. (LUCK OF THE DRAW) But I do strongly believe that good water quality and top-notch food are key to a peaceful reef.